1. Field of the Invention
This invention relates to a simple method for detecting temperature distributions in single crystals and a method for manufacturing silicon single crystals by employing the simple method. More specifically, this invention relates to a simple method for detecting temperature distributions in single crystals within a furnace of single-crystal manufacturing apparatus, in which grown-in defects are required to be controlled, and relates a method for manufacturing silicon single crystals by employing the simple method. The density and characteristics of the grown-in defects are depending on the heat history of the single crystals produced.
2. Description of the Related Art
The substrates of semiconductor components are mainly composed of a high-purity silicon single crystal that is generally produced by the CZ method. In the CZ method, polycrystalline silicon nuggets are fed into a crucible of a semiconductor single-crystal manufacturing apparatus. Then the quartz crucible is heated by cylindrical heaters disposed therearound to melt the polycrystalline silicon lumps, after which a seed crystal installed on a seed chuck is dipped into the molten silicon. After that, the seed chuck and the quartz crucible are respectively driven to rotate in opposite or identical directions, and at the same time the seed chuck is pulled to grow a single-crystal silicon ingot of predetermined diameter and length.
In recent years, with the microminiaturization and the high-density accumulation of the device structures, gate oxide film integrity has become a more important characteristic. The deterioration of the gate oxide film integrity of LSI is caused by grown-in defects, which are formed in silicon single crystals during the crystal growth. It is found that the grown-in defects include LSTD (Laser Scattering Tomography Defects), FPD (Flow Pattern Defects), and COP (Crystal Originated Particle), and it is known that there exists a distinct relationship between the density of the grown-in defects and B-mode failure ratio with respect to gate oxide film integrity.
Grown-in defects are formed in a solidified silicon single crystal during the process of growing it from the molten silicon, at a temperature near 1100.degree. C., specifically, when it passes a temperature range from 1150.degree. C. to 1080.degree. C. It has been established that density of grown-in defects can be reduced if the cooling rate of the single crystal is reduced when the crystal passes through the above temperature range, and if this is achieved then oxidation-film voltage endurance can be enhanced. The cooling rate in this temperature range is a factor for determining the density or the size of Grown-in defects. Accordingly, it is known that the cooling rate is controlled so that the density or the size of Grown-in defect can be changed desirably. Alternatively, it is known that if a single crystal is cooled down rapidly and oxygen precipitation heat treatment is exerted on the single crystal, then AOP (Anomalous Oxygen Precipitate) can be found in specified portions of the single crystal. Therefore, it is very useful to the industry that; if a location where its interior temperature approaches 1100.degree. C. and the cooling rate of a single crystal being pull passing the location thereof are recognized; and then those are used to control the grow-in defect.
The cooling rate of a silicon single crystal being grown is determined by the pulling speed and the temperature distributions in the silicon single crystal (the temperature gradient in the interior of the silicon single crystal). The pulling speed is preset and known and only the temperature distributions in the silicon single crystal need to be calculated. Computer-aided simulation and temperature detection with the aid of thermal couples are also available to ascertain the temperature distributions in a silicon single crystal.
In the process of computer-aided simulation, calculation has to be carried out by simplifying the interior structure of the furnace to some extent, and this will lead to some problems to precision of calculation. Furthermore, when directly detecting the interior temperature of a crystal, it is necessary to prepare thermal couples and a dummy crystal instead of the single crystal to be grown. It is also necessary to choose thermal couples made of high-temperature endurance materials, such as platinum--platinum-rhodium, and the thermal couples has to be sheathed with quartz tubes to enable them to be insulated from the dummy crystal into which the thermal couples are embedded. The thermal couples and the quartz tubes are very expensive and sheathing the thermal couples is a time-consuming process. Usually, a silicon single crystal pulled according to the CZ method is used as a dummy crystal, and holes have to be drilled thereon. In the process of drilling, the dummy crystal is very brittle, and precision is required with this technique.
Furthermore, it is required to connect the thermal couples with a voltmeter disposed outside the furnace. Therefore, the dummy crystal can not be rotated like the single crystals being pulled. Otherwise, it will be impossible for temperature distributions to be detected precisely. Furthermore, wiring of the thermal couples will restrain the upward or downward movements of the dummy crystal, and the movement of the dummy crystal may rupture the wires.